1. Field of the Invention
This invention relates to the electrodeposition of chromium using lead or lead alloy anodes, and, more particularly, to a chromium plating bath system and process, in which weight loss of such lead anodes is substantially reduced.
2. Description of the Prior Art
Chromium plating baths using chromic acid solutions as the source of chromium almost invariably employ lead or lead alloy anode. However, as described in the book "The Electrochemistry of Lead" by A. T. Kuhn, Academic Press (1979) pages 405-407, lead and lead alloy anodes corrode to a soluble species (leading to anode weight loss) in chromic acid solutions. This anode dissolution proceeds at a substantial rate, which is proportional to the acidity of the solution. For this reason, there has been a need to find corrosion resistant alloys for this medium. For example, alloys containing 10% Sn and 0.5% Co seem to show good resistance. Also the addition of 1-2 g/l of Co.sup.2+ ions appears to further reduce corrosion.
While this problem is present in the conventional hexavalent chromium plating baths and in the mixed catalyst chromium baths, it is particularly acute in high energy efficient baths, e.g. the so-called "HEEF-25" baths as described in the U.S. Pat. No. 4,588,481. HEEF-25 baths contain chromic acid, sulfate and an alkyl sulfonic acid, which, for functional chromium deposits, is preferably a non-substituted alkyl sulfonic acid, or salt thereof, wherein the ratio of S/C is .gtoreq.1/3. Typical alkyl sulfonic acids are methyl sulfonic acid, ethyl sulfonic acid, propyl sulfonic acid, methane disulfonic acid and 1,2-ethane disulfonic acid.
Other hexavalent chromium plating baths are described in the following U.S. Pat. Nos.: 2,750,337; 3,310,480; 3,311,548; 3,745,097; 3,654,101; 4,234,396; 4,406,756; 4,450,050 and 4,472,249.
Accordingly, it is an object of the invention to provide a chromium plating bath in which the rate of weight loss or pitting of a lead or lead alloy anode is substantially reduced.
Another object of the present invention is to provide an additive for a hexavalent chromium plating bath containing an alkyl sulfonic acid, which substantially reduces the corrosion of a lead or lead alloy anode therein, which corrosion produces a soluble species leading to anode weight loss.
Still another object of the invention is to provide a method of preventing such deleterious corrosion of a lead or lead alloy when used in a hexavalent chromium plating bath, particularly in a high energy efficient bath.
These and other objects and features of the invention will be made apparent from the following more particular description of the invention.